Gustav Hertz was a nephew of Heinrich Hertz, the namesake of the unit used for characterizing frequencies. He received the 1925 Nobel Prize in Physics jointly with James Franck for showing that atoms possess discrete energy levels. In the years after the award Hertz should slightly shift his research focus and work on methods for the separation of isotopes, amongst other things. Isotopes are versions of a given element featuring the same number of protons and electrons, but a varying number of neutrons and thus a different atomic mass. In the present lecture, Hertz gives a comprehensive and easy-to-follow overview of the use of isotopes in science, covering methods for the isolation of isotopic species, the isotopic distributions encountered in nature, current applications of isotopically labelled compounds in bioanalysis as well as techniques related to isotope quantification, such as mass spectrometry. He furthermore discusses the Pros and Cons of using radioactive (and thus unstable) isotopes versus using stable, non-radioactive variants for research purposes. At the time of the talk, these issues were highly topical and the development of isotope-based methods in science and technology was rapidly proceeding. In the realm of the Nobel Prize, this is exemplified, for instance, by the 1943 Nobel Prize in Chemistry to George de Hevesy "for his work on the use of isotopes as tracers in the study of chemical processes". Hevesy, who may well be considered the founding father of the biomedical application of isotopes, detailed the impressive methods developed in his laboratory in several Lindau talks, e.g. in 1952 and 1955. Artturi Virtanen, Chemistry Laureate of 1945, used isotopically labelled nitrogen to study the nitrogen assimilation by legumes. In the year after Hertz’ talk, a further related Nobel Prize should go to William Libby for the development of radiocarbon dating. Traditionally, radioactive isotopes were favoured for studying biological processes, because the measurement of the associated radioactivity, e.g. in plants, tissue or body fluids, could easily be accomplished. Hertz acknowledges this towards the end of his talk, but still strikes a blow for an increased use of the non-radioactive isotope variants. Stable isotopes offer several advantages, as Hertz points out. They do not decay and do not require specially equipped laboratories or security measures. However, they may not be quantified by radioactivity measurements and hence require advanced analytical techniques, such as mass spectrometry. While Hertz admits mass spectrometry (at the time) to be a complicated and expensive technique, he also points out that the first commercial analysis laboratories had been established.From a today’s perspective, Hertz was clearly on the right track. In the decades after his talk, mass spectrometry developed massively and other techniques for the analysis of isotopic species such as nuclear magnetic resonance spectroscopy (NMR) became available. Today, the instrumental portfolio routinely available to analysts allows not only for the facile quantification of the total quantity of isotope atoms in a given sample, but also for a rapid and straightforward localization of isotopes in the chemical structures of partially-labelled compounds. As a consequence, stable isotopes have largely replaced their radioactive counterparts in the scientific world. David Siegel